1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and more particularly, to apparatuses and methods for adjusting the voltage on a powered Faraday shield to control the behavior of a plasma inside an inductively coupled plasma etching chamber.
2. Description of the Related Art
In semiconductor manufacturing, etching processes are commonly and repeatedly carried out. As is well known to those skilled in the art, there are two types of etching processes: wet etching and dry etching. Dry etching is typically performed using an inductively coupled plasma etching apparatus.
FIG. 1 shows an inductively coupled plasma etching apparatus 100, in accordance with the prior art. The inductively coupled plasma etching apparatus 100 includes an etching chamber 101 structurally defined by chamber walls 102 and a chamber window 104. The chamber walls 102 are typically fabricated from stainless steel; however, other suitable materials may also be used. The chamber window 104 is typically fabricated from quartz; however, other materials such as alumina (Al2O3), silicon nitride (Si3N4), aluminum nitride (AlN), silicon carbide (SiC), and silicon (Si) may also be used. The chamber window 104 provides a vacuum seal to the chamber walls 102. A semiconductor wafer (i.e., “wafer”) 110 is mounted on a chuck 108 positioned on the bottom inner surface of the etching chamber 101. A coil 116 and a metal shield 112 are positioned above the chamber window 104. The coil 116 is held above the etching chamber 101 by insulating spacers (not shown). The coil 116 is fabricated from an electrically conductive material and includes at least one complete turn. The exemplary coil 116 shown in FIG. 1 includes three turns. The coil 116 symbols having an “X” indicate that the coil 116 extends rotationally into the page. Conversely, the coil 116 symbols having a “●” indicate that the coil 116 extends rotationally out of the page. The metal shield 112 is secured beneath the coil 116 in a spaced apart relationship using insulating spacers 114. The metal shield 112 is disposed immediately above the chamber window 104. The coil 116, the metal shield 112, and the chamber window 104 are each configured to be substantially parallel to one another. Furthermore, the coil 116 and the metal shield 112 are electrically connected through a tap 118.
FIG. 2 shows the basic operating principles of the inductively coupled plasma etching apparatus 100, in accordance with the prior art. During operation, a reactant gas flows through the etching chamber 101 from a gas lead-in port (not shown) to a gas exhaust port (not shown). High frequency power (i.e., RF power) is then applied from a power supply (not shown) to the coil 116 to cause an RF current to flow through the coil 116. The RF current flowing through the coil 116 generates an electromagnetic field 120 about the coil 116. The electromagnetic field 120 generates an inductive current 122 within the etching chamber 101. The inductive current 122 acts on the reactant gas to generate a plasma 123. High frequency power (i.e., RF power) is applied from a power supply (not shown) to the chuck 108 to provide directionality to the plasma 123 such that the plasma 123 is “pulled” down onto the wafer 110 surface to effect the etching process.
The plasma 123 contains various types of radicals in the form of positive and negative ions. The chemical reactions of the various types of positive and negative ions are used to etch the wafer 110. During the etching process, the coil 116 performs a function analogous to that of a primary coil in a transformer, while the plasma 123 performs a function analogous to that of a secondary coil in the transformer.
The reaction products generated by the etching process may be volatile or non-volatile. The volatile reaction products are discarded along with used reactant gas through the gas exhaust port. The non-volatile reaction products, however, typically remain in the etching chamber 101. The non-volatile reaction products may adhere to the chamber walls 101 and the chamber window 104. Adherence of non-volatile reaction products to the chamber window 104 may interfere with the etching process. A deposition of electrically conductive non-volatile reaction products on the chamber window 104 may electrically shield the inner region of the etching chamber 101 from the electromagnetic field 120 generated about the coil 116. Consequently, the plasma 123 will not strike well, and the etching process will have to be discontinued until the deposition is removed from the chamber window 104. Additionally, excessive deposition may result in particles flaking off the chamber window 104 onto the wafer 110, thus interfering with the etching process. Excessive deposition, therefore, requires more frequent cleanings of the etching chamber 101 and the chamber window 104.
Deposition of non-volatile reaction products on the chamber window 104 can be mitigated and prevented by sputtering the plasma against the chamber window 104 to “knock off” the deposition. To avoid non-uniformity in the plasma 123, the sputtering should be performed in a uniform manner across the chamber window 104. Non-uniform deposition and/or non-uniform sputter can introduce drift into the etching process. Drift can prevent reproducibility among a number wafers 110 whose characteristics are intended to be uniform.
The metal shield 112 acts as a Faraday shield to ensure that electromagnetic energy generated by the coil 116 is uniformly distributed to the plasma 123. As a result of uniformly distributing the electromagnetic energy to the plasma 123 in the vicinity of the chamber window 104, the deposition of non-volatile reaction products onto the chamber window 104 will occur uniformly. Similarly, the sputtering of non-volatile reaction products from the chamber window 104 will also occur uniformly. The presence of uniform electrical characteristics across the chamber window 104 facilitates the generation of uniform plasma 123 characteristics across the etching chamber 101. However, even uniform deposition of non-volatile reaction products on the chamber window 104 will continue to interfere with the etching process as previously discussed. Therefore, it is necessary to sputter the plasma 123 against the chamber window 104 to prevent buildup of non-volatile reaction product deposition. The sputtering of plasma 123 against the chamber window 104 must be performed carefully to minimize or prevent erosion of the chamber window 104 by the charged particles of the plasma 123.
FIG. 3 shows how the Faraday shield voltage can be controlled to affect the chamber window 104 characteristics, in accordance with the prior art. View 134 shows the effects of applying an appropriate voltage to the metal shield 112 to control deposition and sputter of non-volatile reaction products relative to the chamber window 104. With the appropriate voltage applied to the metal shield 112, incident ions 128 of the plasma 123 will be uniformly directed toward the chamber window 104. The energy and intensity of the incident ions 128 will prevent deposition while simultaneously minimizing the erosive effects of sputtering. View 136 shows the effects of applying too low a voltage to the metal shield 112. With too low a voltage, incident ions 130 directed toward the chamber window 104 will lack the required energy and intensity to prevent buildup of non-volatile reaction products commonly called a deposition 124. View 138 shows the effects of applying too high a voltage to the metal shield 112. With too high a voltage, incident ions 132 will be directed toward the chamber window 104 having too much energy and too much intensity, thus causing too much sputtering. Excessive sputtering can result in erosion 126 of the chamber window 104. Such erosion 126 not only shortens the lifetime of the chamber window 104, but also generates particles which can contaminate the wafer 110 and introduce unwanted chemical species into the etching process environment. The presence of unwanted species in the etching process environment is particularly undesirable because it leads to poor reproducibility of the etching process conditions.
The appropriate Faraday shield voltage is dependent on the particular etching process that is being performed. Some factors that may influence the appropriate voltage include the type of reactant gas, the RF power applied to the coil 116, the material to be etched from the wafer 110, and the process environment conditions inside the etching chamber 101. Many etching recipes include multiple etching steps, e.g., the breakthrough step, the bulk etch steps, and the over etch step, in which the RF power, pressure, and gas compositions can be substantially different. Consequently, a certain setting for the appropriate Faraday shield voltage for a given etch step may not be optimal in other etch steps. Thus, the Faraday shield voltage should be controllable to ensure that the chamber window 104 is maintained free of non-volatile reaction product deposition during the etching process. Furthermore, the Faraday shield voltage should be easily adjustable to accommodate the variance in voltage requirements for different etching processes and steps. In the prior art, the etching apparatus has been mechanically reconfigured to obtain an appropriate Faraday shield voltage for a particular etching process. Such mechanical reconfiguration has a narrow operating window and is costly in terms of material expense and time resulting in lower wafer throughput.
In view of the foregoing, there is a need for an apparatus and a method to easily adjust the voltage applied to a Faraday shield of an inductively coupled plasma etching apparatus.